Mechanism of Pit Initiation on Aluminum Alloy Type 7075

The influence of anions on the initiation of pitting and the kinetics of pit growth on aluminum alloy Type 7075 has been investigated. The order of reaction (i.e., the number of anions per Al surface reaction site) and the energy of activation for pitting initiation have been measured in F⁻, Cl⁻, Br⁻, and I⁻ solutions. The technique involved potentiostating the alloy in the range of anodic passivity, injecting a solution of halide ion, and recording the time, τ, at which an exponential increase in corrosion current occurs. The slope of the plot of the logarithm of vs. the logarithm of the concentration gives, , the number of anions associated with an Al surface reaction site in the primary pitting initiation step. The value of in varies with the anion as: for Cl⁻, ; for Br⁻, ; and for I⁻, . In an electrolyte of pH 5.8 the value for Cl⁻ is 2; for Br⁻, 2; and for F⁻, 3. The activation energies measured over the temperature range 5°–60°C in were: for Cl⁻, ; for Br⁻, 26 kcal mole⁻¹; and for I⁻, 6.6 kcal mole⁻¹. In an electrolyte of pH 5.8, the is 12 kcal mole⁻¹ for Cl⁻; 10 kcal mole⁻¹ for Br⁻; and 4.6 kcal mole⁻¹ for F⁻. These results are interpreted in terms of the formation of transitory species akin to activated complexes, of specific composition, such as .